The present invention generally relates to a radio frequency (RF) coil arrangement used in a magnetic resonance imaging apparatus (MRI apparatus) for detecting magnetic resonance signals (MR signals) emitted from hydrogen and phosphor atoms contained in an object under examination such as a human body, and for imaging a density distribution and a relaxation time distribution of nuclei spins contained in the object under examination. More specifically, the present invention is directed to such an RF coil arrangement used in a vertical magnetic field type MRI apparatus in which a direction of a static magnetic field is a vertical direction with respect to a body axis of the object under examination.
MRI apparatuses are employed so as to acquire tomographic images of objects under examination by utilizing the MRI phenomenon. That is, pulsatory RF magnetic fields are applied to an object under examination positioned in a static magnetic field, so that the nuclear magnetic resonance (NMR) phenomenon occurs in nuclei spins such as hydrogen for constituting a tissue of the object under examination. Then, MR signals corresponding to electromagnetic waves emitted from the nuclei spins are detected to reconstruct a tomographic image by using the detected MR signals.
In MRI apparatuses, RF coil arrangements are utilized so as to apply RF pulses and also detect MR signals. As an RF coil arrangement used in a horizontal magnetic field type MRI apparatus whose static field direction is located in parallel to a body axis direction of an object under examination, there are used a solenoid coil; various sorts of quadrature detection (QD) coils, which may surround a region of interest of this object under examination; and either an array coil or a QD array coil used only to a specific portion of this object under examination. The QD coil is known from the QD method of "Journal of Magnetic Resonance" volume 54, in 1983, pages 324 to 327. In this QD method, the linear magnetic field signals are detected by two sets of coils positioned perpendicular to each other, and then the phases of the linear magnetic field signals detected by these two coils are mutually shifted by 90 degrees. Thereafter, the phase-shifted signals are combined with each other, so that the rotation magnetic field signals are detected in a high sensitivity. In such an ideal case that the strengths of the signals detected by the two coils located perpendicular to each other are equal to each other, the resultant sensitivity can be increased by root of 2 times higher than that of a single linear magnetic field detecting method. On the other hand, the array coil is described in "Magnetic Resonance in Medicine" volume 16, in 1990, pages 192 to 225, and U.S. Pat. No. 4,825,162, namely the phased array coil techniques. Generally speaking, a small sized coil owns a relatively higher signal receiving sensitivity than a signal receiving sensitivity of a large sized coil. It should be noted that a signal receiving sensitivity region of such a small sized coil becomes narrower than that of the large sized coil. On the other hand, the array coil described in the publications is arranged by a plurality of small-sized unit coils having high sensitivities in such a manner that interference produced between the adjacent unit coils becomes minimum. Accordingly, the field of this array coil (namely, high sensitivity region of array coil) can be essentially enlarged while maintaining the high sensitivity.
With respect to the QD coil for the vertical magnetic field type MRI apparatus, for instance, JP-A-3-268744 and JP-A-5-317284 describe the head portion imaging reception coil and the abdominal portion imaging reception coil, which are constituted by combining the solenoid coil with the saddle coil.
Also, JP-A-6-14901 and "SNR and Noise-Correlation of CP-volume-Array-Coils" written by L. Kreischer et al, Proceeding of Society of Magnetic Resonance, in 1995, page 980 have proposed the horizontal magnetic field type QD array coil in which a plurality of rectangular coils and a plurality of coils having a shape of FIG. 8 are arranged.
The above-described conventional QD coil used in the vertical magnetic field type MRI apparatus owns the following problems. That is, since two sets of coils for constituting this QD coil own shapes for surrounding the region of interest of the object under examination, the sensitivities of these coils are lowered due to the adverse influences caused by the object under examination in such a middle field strength (for example, 1 Tesla) MRI apparatus. Thus, the image quality of the resultant tomographic images is not so high. Even in such a low field strength (for example, 0.3 Tesla) MRI apparatus, when this QD coil is used as the coil only for a knee and a foot neck of a human body, a region of interest is fixed by this QD coil. Accordingly, it is practically difficult to image the human body while varying the attitude of this human body, for instance, bending or expanding. On the other hand, the above-explained conventional QD array coil is designed for the horizontal magnetic field type MRI apparatus, but not intended to be used in the vertical magnetic field type MRI apparatus.